In power electronic circuits such as inverters, converters, etc., power semiconductor switches such as MOSFETs (metal oxide semiconductor field effect transistors), IGBTs (insulated gate bipolar transistors), and JFETs (junction field effect transistors) are controlled through a control electrode such as a gate electrode for MOSFETs, gate electrode for IGBTs, base current electrode for bipolar transistors, etc. Commands for controlling turn-on, turn-off, blocking and conducting states of the power semiconductor switches are generated in a controller and transferred to the control terminal by gate drivers for each power switch. The gate drivers shift the command signals from the controller input voltage (e.g. via a transformer, opto-coupler, level-shifter, etc.) and shape the drive signals for intended switching transitions (slope, rise and fall time, delay time, etc).
Power semiconductor devices as mentioned above can also be used to manage fault conditions e.g. by detecting short circuit of loads. A load short circuit can occur between two phases, all three phases or between one or more phases to ground. Under such short circuit conditions the output characteristics of the power semiconductors are utilized. For example, the drain (collector) current i.e. the current between the power terminals of the power semiconductor device saturates at about 4 to 10 times the rated current, whereas the saturation level is determined by the amount of gate voltage and the transfer characteristic of the device. Power semiconductors can withstand such high current conditions at high voltage for only few μs. The driver or controller senses such conditions quickly and turns off the power semiconductor device. Different types of short circuit conditions may arise. In each case, the characteristic di/dt and dV/dt response of the power circuit causes gate overvoltage conditions which arise because of the stray inductance seen at the gate input of the device. This stray inductance, generally referred to herein as the gate circuit inductance, includes the inductance associated with wiring on the board of the gate driver (board layout), the wiring from the gate driver to the control terminals of the power module, and terminals, wires and conductor paths inside the power module to the power transistor gates. The height of the gate overvoltage depends in part on the gate circuit inductance. In other words the speed at which charge at the gate can flow into the voltage source of the driver is not only limited by the resistance of the gate circuit, but also by the inductance. The gate circuit inductance limits the speed by which gate current can change. Therefore, for typical gate circuits, gate over voltages can exceed 20 V which is normally the maximum rating.
Also in modules for higher power chips which are paralleled together, the common gate requires a more powerful gate driver. Within the gate driver this is usually achieved by the use of transistors having higher current ratings and lower gate resistors. The wiring on the board of the gate driver (board layout), the wiring from the gate driver to the control terminals of the power module, and terminals, wires and conductor paths inside the power module to the gates of the paralleled devices remains similar as in the single transistor case. This yields a gate circuit inductance which is about the same as in the single transistor case. The current out of the common gate scales up with the number of devices in parallel. The di/dt of the gate current also scales up, accordingly, which causes higher overvoltage at the gate. The gate circuit inductance is a function of the geometry in the driver circuit and the connections to the power transistor module. A lower gate circuit inductance improves short circuit response which aids in quickly limiting the gate voltage to the value set by the driver and consequently limiting short circuit current at a load followed by a controlled turn-off of the short circuit (the main problem with high inductance in the gate circuit is the increase of gate voltage during a short circuit condition). A lower gate circuit inductance also improves the turn-on and turn-off response of power transistor devices, providing a faster device response time. Gate circuit inductance is conventionally overlooked in favor of resistive impedance. Gate circuit inductance has been addressed by the assembly of gate driver boards directly onto power module terminals without wiring in between. The inductance on the gate driver board and inside the power modules or packages is not typically addressed.